Vinyl alcohol polymers (hereinafter, may be abbreviated as “PVAs”) typified by polyvinyl alcohol are known as water-soluble synthetic macromolecules, and are widely used in intended usages including raw materials for vinylon, which is a synthetic fiber, paper coating agents, fiber coating agents, adhesives, stabilizers for emulsion polymerization and suspension polymerization, binders for inorganic substance, and films. In particular, PVA is superior in strength characteristics and film-forming property as compared with other water-soluble synthetic macromolecules, and based on these properties, PVA is successfully utilized as coating agents for improving surface properties of substrates such as papers (clear coating agents, binders in pigment coating, coating agents (sealing agent) for a paper surface of release papers, and the like), materials for forming an ink-receiving layer in ink jet recording materials, materials for forming a coating layer such as a thermal coloring layer or an overcoat layer in thermal recording materials.
In order to further enhance the characteristics of such PVA, various types of modified PVAs have been developed. As one example of the modified PVAs, silyl group-containing PVA is exemplified. The silyl group-containing PVA has high water resistance and binder force to inorganic substances. However, the silyl group-containing PVA is accompanied by the following disadvantages that: (a) in the preparation of aqueous solutions of thereof, the silyl group-containing PVA is less likely to be sufficiently dissolved unless an alkali such as sodium hydroxide or an acid is added; (b) viscosity stability of the prepared aqueous solutions tends to be readily deteriorated; (c) in the formation of a coating film containing inorganic substances, it is difficult to obtain a coating film having the water resistance and the binder force to the inorganic substance both being satisfactory; and the like.
In addition, when an inkjet recording material is produced using such silyl group-containing PVAs in an ink-receiving layer, a disadvantage is observed that strength and water resistance of the coating film are insufficient, and unevenness (printing unevenness) is likely to be caused in printing.
Further, thermal recording materials produced using the PVA in the coating layer fail to exhibit water resistance and resistance to plasticizers sufficient to satisfy a high level of requirements for thermal recording materials in recent years.
In addition, sealability (low gas-permeability and/or superior solvent-barrier properties) and water resistance of the coating film obtained from an aqueous solution of the silyl group-containing PVA are insufficient.
In light of the foregoing, there have been proposed a silyl group-containing PVA having an improved solubility in water and the like by adjusting a product (P×S) of a viscosity average degree of polymerization (P) and a content (S: mold) of monomer units having a silyl group to fall within a predetermined range, and the like (see Japanese Unexamined Patent Application, Publication No. 2004-43644), a coating agent containing the silyl group-containing PVA, an ink jet recording material produced from the coating agent, and a thermal recording material (see Japanese Unexamined Patent Application, Publication No. 2005-194437), and a base paper for a release paper including a coating layer containing a silyl group-containing PVA that has improved barrier properties and water resistance and the like by adjusting a product (Pw×S) of a weight average degree of polymerization (Pw) and a percentage content (S: mol %) of monomer units having a silyl group to fall within a predetermined range, and the like (see Japanese Unexamined Patent Application, Publication No. 2005-194672). However, in the silyl group-containing PVAs, the upper limit of the product (P×S) has been proposed to be 370, and a trade-off relationship between enhancing the characteristics as the silyl group-containing PVA through an increase of the content of the monomer units having a silyl group and enhancing a solubility in water and the like thereof has not been overcome. More specifically, the silyl group-containing PVA has a disadvantage in handling that in a case where the product (P×S) is 370 or greater, in preparing an aqueous solution of the silyl group-containing PVA, it is occasionally impossible to dissolve the silyl group-containing PVA unless an alkali or an acid is added, as disclosed in paragraph 0009 of Japanese Unexamined Patent Application, Publication No. 2004-43644 cited above. Accordingly, the silyl group-containing PVA also has not sufficiently solved the aforementioned disadvantages.